1. Field of the Invention
The present invention relates to a magnetoresistive effect element used as a magnetic sensor.
2. Description of the Related Art
As reproducing heads for reading information from a magnetic medium, a magnetoresistive (MR) sensor or head using a magnetoresistive effect is known. The MR sensor operates in accordance with an anisotropic magnetoresistive effect: the resistance of a magnetoresistive layer is dependent upon the square of the cosine of an angle between a magnetization direction and a sense current direction of a ferromagnetic layer (see: David A. Thompson et al., "Thin Film Magnetoresistors in Memory, Storage, and Related Applications", IEEE Trans, on Magnetics, Vol. Mag-11, No. 4, PP. 1039-1050, July 1975).
On the other hand, laminated MR sensors, i.e., artificial lattice structured MR sensors, have recently been suggested to enhance the anisotropic magnetoresistive effect, which is called a giant magnetoresistive effect or a spin valve effect. That is, a magnetoresistive multi-layer is formed by a pair of ferromagnetic layers separated by a nonmagnetic layer. In this case, a resistance of the MR multi-layer is dependent upon an angle between magnetization directions of the adjacent ferromagnetic layers, to obtain a large resistance change.
In a first prior art laminated MR sensor (see: JP-A-2-61572), the ferromagnetic layers are made of appropriate materials such as ferromagnetic transition metal or its alloy, so that one of the ferromagnetic layers has an opposite magnetization direction to that of the other ferromagnetic layers in accordance with the application or nonapplication of an external magnetic field. Also, the thickness of the nonmagnetic layer is smaller than the conductive electron mean path. Thus, a larger magnetoresitive effect can be obtained.
In a second prior art laminated MR sensor (see JP-A-4-358310), when a magnetic field is not applied thereto, a magnetization direction of one of the ferromagnetic layers is perpendicular to a magnetization direction of the other, so that a resistance between the ferromagnetic layers is dependent upon the cosine of an angle between the magnetization directions of the ferromagnetic layers independent of a sense current flowing therethrough.
Generally, an output V.sub.s of an MR sensor (magnetoresistive element) is represented by EQU V.sub.s =I.sub.s .multidot.R
where I.sub.s is a definite sense current; and
R is a resistance of the magnetoresistive element. Therefore, the difference .DELTA.V.sub.s is EQU .DELTA.V.sub.s =I.sub.s .times..DELTA.R
where .DELTA.R is a change of the resistance of the magnetoresistive element. Thus, in order to increase the sensitivity of the magnetoresistive element, one approach is to increase the change of the resistance, and the other approach is to increase the definite sense current I.sub.s.
Without changing the materials of the magnetoresistive element, it is known for the change of the resistance to be increased by improving the crystalline characteristics and crystalline orientation characteristics of the magnetoresistive element. However, since the thickness of the entire magnetoresistive element is very thin, for example, about 10 to 30 nm, it is substantially impossible to improve the crystalline characteristics and the crystalline orientation characteristics.
Also, in the multi-layer structured MR sensors, a heat process during a photolithography process creates diffusion between the ferromagnetic layers and the nonmagnetic layers, which reduces the property of the magnetoresistive element. For example, if each of the ferromagnetic layers is made of a NiFe layer, and each of the nonmagnetic layers is made of a Cu layer, Cu atoms penetrate into grain boundaries of the NiFe layer, and Ni atoms and Fe atoms penerate into grain boundaries of the Cu layer.
On the other hand, in a high density magnetic recording medium, the pitch of tracks is very small, so that the pitch of heads of the MR sensors therefor is also very small. That is, the magnetoresistive element is small in size. In this case, in order to maintain the sensitivity of the magnetoresistive element, a current density of the sense current I.sub.s has to be increased, i.e., to about 5.times.10.sup.5 to 1.times.10.sup.8 .ANG./cm.sup.2. As a result, electromigration due to high electric field occurs in the magnetoresistive element which destroys the magnetoresistive element.